Apple Pay on Web using mass enablement returns 417 error

We are a PSP and use Apple Pay Mass Enablement certificates for our merchants to provide Apple Pay as a payment method via our Web SDK integration.

We pass on the domain registration file that we received from the Apple Pay Support team when we initially integrated Apple Pay on Web to our merchants to place in their /.well-known. The merchant places this file in their /.well-known location and then we are able to register them using our Judopay Merchant ID.

We have recently tried to register new merchants and it seems like they are registered fine (as per the 200 response from Apple Pay Register merchant API).

However, we receive this error when trying to initiate an Apple Pay session -

ResponseBody: { "statusMessage": "Payment Services Exception merchantId=FFDAC5CDD0891FAF5B23AC57562C268CA22376480B04D3295C437F5B13C80985 unauthorized to process transactions on behalf of merchantId=80FCA6BC3E3A7463115CCC9F5634ACEC99CF1CCA628F34FD22F2C7873410F903 reason=80FCA6BC3E3A7463115CCC9F5634ACEC99CF1CCA628F34FD22F2C7873410F903 is not a registered merchant in WWDR and isn't properly authorized via Mass Enablement, either.", "statusCode": "417" }

We have followed a set of troubleshooting steps and checked that the merchant identity certificate, merchant id, payment processing cert and so on are valid and not expired. We can not find an appropriate resolution, has anyone else experienced this? Is there a known reason why this might occur?